In underwater photography the range of visibility is often limited and many of the subjects photographed are at very close range. Cameras with variable focus which can focus at very close range are usually enclosed in a sealed housing, which is expensive and cumbersome. Due to problems of sealing a camera for underwater use without a protective housing, the range of adjustments, such as for focus, are usually very limited.
To allow a camera to focus at very close range an adapter in the form of a lens extension has been used. This fits between the camera body and the lens and moves the lens forward to increase the lens to film plane plane distance, which shortens the focal distance. To facilitate framing and alignment of the camera with the object being photographed, a guide frame of wire or the like is mounted on the camera on an extension arm of the proper length, so that an object positioned within the guide frame is correctly in focus.
A problem with an underwater camera, with or without a lens adapter, is that the pressure differential between the interior of the camera and the surrounding water can cause distortion of the camera body and change the effective focal length of the lens. Since at very close ranges the depth of field is very limited, even a slight distortion can cause a reduction in sharpness of the image. For precise results it is therefore necessary to compensate for pressure differential, without elaborate housing or sealing structure.